The Printed Circuit Board (PCB) is a type of important electronic component, and, in general, used to achieve electrical connection among various electronic components. In the PCB with multi-layers, electrical signal is transmitted among various conductive layers of the PCB via a through hole disposed in the PCB. For example, the electrical signal may be transmitted between wirings on two conductive layers of the PCB via the through hole. As shown in FIG. 1, the PCB includes two core boards a1 and a2 and two prepregs b1 and b2, wherein core board a1 includes two conductive layers a11 and a13 and an insulating layer a12, and wherein core board a2 includes two conductive layers a21 and a23, and an insulating layer a22. The through hole c of the PCB has an inner wall plated with a conductive material c1, which can transmit the electrical signal between the wiring on the conductive layer a11 and the wiring on the conductive layer a13 of the PCB.
In a process of manufacturing the PCB, after lamination of the PCB, the through hole is formed by a high speed drilling tool at a preset position on the PCB. The through hole is then metalized, that is to say, the inner wall of the through hole is deposited with copper and then is plated, so that the electrical signal may be transmitted among various conductive layers. In some of those PCBs, however, the through hole is merely used to transmit the electrical signal between the wirings on some of the conductive layers. For example, in the PCB as shown in FIG. 1, the through hole c is merely used to transmit the electrical signal between the wiring on the conductive layer a11 and the wiring on the conductive layer a13, while portions of the through hole, which is throughout the prepregs b1 and b2 and the insulating layer a22, are not necessary for transmitting the electrical signal, and thus form a short line d (the short line refers to extra conductive materials in the through hole, which is not necessary for transmitting the electrical signal).
When the high speed electrical signal is transmitted via the through hole, it is easy to distort the electrical signal during transmission due to one or more short lines existing in the through hole in the PCB, which is called short-line effect. With the short-line effect, when the electrical signal is transmitted via the through hole, a part of the electrical signal will enter into one or more short lines of the through hole from a wiring connection on the conductive layer. This part of the electrical signal then may be reflected from an end of the short line to the wiring connection after a certain delay. This delayed reflection may make interference of the integrity of the electrical signal, and increase bit error rate of the electrical signal. In addition, attenuation of the electrical signal will increase as the length of the short line increases. As a result, in order to ensure the transmission of the electrical signal, the short-line effect should be eliminated.
Currently, in order to remove the short line of the through hole c in the PCB, the short line of the through hole is back drilled using a drilling tool in general. As shown in FIG. 2, a PCB which is back drilled to remove the short line d of FIG. 1 is shown. A back-drilled hole h is formed at the position corresponding to the short line d as shown in FIG. 1 after the back-drilling. Although a part of the short line is removed by back drilling, the short line cannot be removed completely since the drilling tool has a pointed tip. Then, parasitic capacitance, parasitic inductance and time delay affecting the integrity of the electrical signal cannot be eliminated completely, that is to say, the short-line effect cannot be eliminated completely. When the PCB is back drilled by using the drilling tool, it is necessary that the drilling tool must have relatively high accuracy. Since the accuracy of the drilling tool is not high, such as, the pointed tip of the drilling tool is too deep accidentally or off-center, it is easy to damage the functional portions (which function to transmit the electrical signal among different conductive layers) of the through hole while removing the short line, so that the PCB is scrapped. As a result, it causes a reduced yield but increased manufacturing cost for the PCB.
In view of the above, in the traditional method of removing the short line from the through hole in the PCB by a way of back-drilling, the short-line effect cannot be eliminated completely, and the PCB tends to be scrapped in the process. Consequently, it causes the reduced yield but increased manufacturing cost for the PCB.